Heidi Murkoff's latest baby

Heidi Murkoff looks like a miniature, bird-like prom queen. But despite appearances the 49-year-old is the "powerhouse mom" behind What to Expect When You're Expecting, the best-selling pregnancy book in the world. It has 16m copies in print in 40 countries, and USA Today reported that 93% of expectant mothers in America read it. But it provokes strong feelings among women. TV presenter Mariella Frostrup described it as her "bible", while the novelist Rachel Cusk denounced it as "frighteningly sanctimonious".

Murkoff, based in South California, is in the UK to promote the latest in her series of spinoffs (which includes an iPhone app), What to Expect Before You're Expecting – a guide to conception. When we meet at a central London hotel, she adopts a gossipy tone: "It all started when I was pregnant for the first time and didn't have the slightest idea. I was the first in my peer group to get pregnant. All I craved was reassurance. I needed someone to tell me that all the seemingly random symptoms I had – weird things, such as excess saliva – were normal. And I was worried because I wasn't getting any morning sickness. When I wrote it I was on a mission to help other parents sleep at night."

But that wasn't the only reason the 23-year-old advertising copywriter – as she was then – felt compelled to deliver a proposal for an encyclopaedic guide to pregnancy two hours before going into labour with her first child. This was "an 'oops' pregnancy," she says. "So . . . I had had a few drinks. I had had cocktails. I had drunk coffee. I panicked. I was six weeks pregnant when I found out." More worrying, her doctor, unaware that Murkoff was pregnant, had prescribed a progesterone tablet to bring on her "irregular" period. Luckily, it didn't work.

The books are meticulously compiled by Murkoff and a team of medical writers to contain every up-to-date nugget of research imaginable. And the UK edition is anglicised a little by a "British childcare expert". Nonetheless, Louise Silverton, deputy general secretary of the Royal College of Midwives, wonders how useful What to Expect really is for British mothers: "Many American books don't translate because of the different healthcare models. The Americans have very few midwives and no post-natal care once you've gone home from hospital."

But Murkoff obviously has the formula right. On a tour in India recently, she received notes under her hotel room door from grateful fathers. In China and Russia they love her. "People do call it a 'bible' and I can understand that. It's indispensable and it's always by your side. When I tour the Bible belt in the US, the mothers call it 'my other Bible'."

Women all over the world trust What to Expect, she says, because it's "the voice of a mom . . . When you open a newspaper you are bombarded with information. There is a lot of scary news, bad news. And yet there has never been a safer time to reproduce. Perspective is helpful." But some of the book's critics have argued that What to Expect just ramps up the pressure on pregnant women to be impossibly perfect. While we talk, Murkoff sips water. I have ordered a latte but only end up drinking half because I am four months pregnant, and Murkoff reminds me I should not be having more than two espresso shots a day.

We differ strongly over alcohol too. I don't think the odd half-glass is harmful, but Murkoff can't agree: "Total abstinence is best because it's easy to blur the line. Susceptibility is something you can't predict. There is no known safe limit. What's the downside to abstaining? I have to go with the party line on this one."

She does, however, make one concession: "I think at times the diet [in the book] was way too stringent. It had people running to the nearest McDonald's because they could not possibly live up to it." (Guilty.) "Now it just has a healthy eating plan." This turns out to be a 32-page guide which advises rice cakes over crisps and edamame beans over pretzels. She warns: "Eating more calories than you and your baby need isn't only unnecessary, it isn't smart – and can lead to excessive weight gain." I dread to think what the first edition's diet tips were like.

And with the pre-conception book, the anxiety has shifted on to the not-yet pregnant too. Murkoff is evangelical about the idea that a pregnancy lasts "at least 12 months" (including the three months before conception). "Taking pre-natal vitamins can minimise birth defects and can lower the chances both of morning sickness and premature delivery," she enthuses. It's so much plotting and worrying, though, isn't it? "Healthy bodies make healthy babies so you have nothing to lose! Is there a downside to eating well and getting in shape?" And at least you're not alone in this project, she beams. "A dad's contribution to conception is every bit as important as the mother's. Brightly coloured fruit and veg make sperm swim faster. And when you're making a baby, speed counts for those boys!" Sound advice, I'm sure.